1. Technical Field
The embodiments described herein relate to a period signal generator, and more particularly, to a refresh period signal generator with a digital temperature information generation function.
2. Related Art
In general, a semiconductor memory device records data in a memory cell or outputs the data recorded in the memory cell to the an exterior of the semiconductor memory device. In addition, the semiconductor memory device has an operation mode called ‘refresh’ that should be essentially performed in order to prevent the data recorded in the memory cell from being lost. The refresh may be classified into a self refresh that is self-performed interior to the semiconductor memory device and an auto refresh that is performed according to a command exterior to the semiconductor memory device.
For example, the auto refresh is performed only in the case when the command is input from the exterior of the semiconductor memory device, while the self refresh is periodically performed in the interior of the semiconductor memory device. Accordingly, the semiconductor memory device requires a period signal for determining the timing of a self refresh operation. The period signal is called a refresh signal, and is generated and used in the semiconductor memory device.
A method for varying the period of the refresh signal depending on a surrounding temperature is adopted to improve the efficiency of the self refresh operation. For this, a device called a Temperature Compensated Self Refresh (TCSR) Oscillator is used.
FIG. 1 is a graph demonstrating a conventional temperature/period output of a self refresh signal. In FIG. 1, the TCSR oscillator generates a pulse signal by increasing the period of the pulse signal as a temperature decreases. The semiconductor memory device uses the generated pulse signal as the refresh signal. The TCSR oscillator enables the efficient self refresh operation by periodically varying the refresh signal.
However, as shown in FIG. 1, the TCSR oscillator generates a refresh signal having an excessively increased period at a predetermined temperature, for example 37° C. or lower (generally called a room temperature or a low temperature in a semiconductor circuit technology), thereby causing a serious error in which the self refresh operation itself is not performed.
The auto refresh operation also requires information on the temperature in the vicinity of the semiconductor memory device in order to use the method for varying the period of the auto refresh signal depending on the surrounding temperature. Refresh rate according to temperature information output through pads DQ8 to DQ10 of a DRAM is defined in the Joint Electron Device Engineering Council (JEDEC) specification relating to a mobile DRAM. However, up to now, hardware and software for providing the temperature information in the semiconductor integrated circuit (IC) have not yet developed.